UH Mānoa physicists make research breakthrough in China

A team of scientists that includes physicists from the University of Hawai‘i at Mānoa earlier this month made a key breakthrough in research at the Chinese Academy of Science‘s Institute of High Energy Physics in Beijing. On July 19, researchers produced for the first time collisions in the upgraded BEPC-II electron positron collider that were observed in its brand new associated detector called BES-III. Although BEPC-II and BES-III had already been carefully tested separately, this was the first time they operated together. These first collisions represent a major milestone of this project, which involved eight years of planning and construction.

The High Energy Physics Group at UH Mānoa‘s Physics Department has been a key group in the BES experiment since 1993. This experiment grew out of the US/PRC Joint Committee on High Energy Physics, which was established 29 years ago and is the oldest scientific cooperative effort between the US and China.

Members of the UHM team include Profs. Fred Harris, Stephen Olsen and Gary Varner. Postdoctoral fellows are Chengping Shen and Qian Liu. For BESIII, UH participants constructed a monitoring system for the Time of Flight (TOF) system, which is used along with the momentum measurement to determine the velocities of the particles coming from the decays of the charmed quarks. The system uses a laser to illuminate a very intricate fiber optic distribution system that sends pulses to the TOF counters. This system was built in Hawaii and shipped last fall where it was installed into BESIII.

The China program greatly increases the capabilities for precision research in this energy region at a time when this type of research at most US facilities has been shut down.

When it is fully operational, the BEPC-II/BES-III complex will be the world‘s premier facility for studying the properties of particles that contain a charmed quark (c-quark), the fourth of an assortment of six different quarks that physicists have identified as the most fundamental building blocks of matter. In BEPC-II, c-quarks, which have a mass that is about 3000 times that of the electron, are produced together with their equal-mass antimatter counterpart, anti-charmed quarks (c-quarks), in head-on collisions of high energy electrons and anti-electrons (familiarly known as positrons). In these collisions, the electron and positron annihilate each other and in the process their energy is converted into the massive c- and c-quark pair in accordance with Einstein‘s famous relation E=mc2.

To accomplish this, the BEPC-II team confines a tightly bunched cluster of approximately 50 billion electrons inside a vacuum tube that threads through a ring of powerful electro-magnets that maintains the electron bunch in a nearly circular orbit. Likewise a similar "bunch" of positrons is made to counter-rotate in an identical second ring of magnets. The two bunches, which have a vertical profile of only about five millionths of a meter, are made to cross each other in the center of the BES-III detector. Occasionally, an electron in one bunch hits a positron in the other bunch head-on and the two particles annihilate each other to produce a pair of particles: one containing a c-quark and an associated one that contains a c-quark. These so-called charmed particles rapidly decay into more conventional particles like - and K-mesons whose energies and velocities are precisely measured in the BES-III spectrometer. From these measurements, the properties of the parent charmed particles can be deduced.

In the July 19 initial test run, a pair of charmed particles, where one contains a c- quark and the other a c-quark was recorded in the detector approximately every ten minutes. The collision rate in the initial test run was about a factor of 4,000 times slower that the project‘s ultimate design goal of 6 or 7 charmed-particle pairs per second. This lower rate was partly because the researchers purposely limited the intensity of the electron and positron beams in order to avoid possible damage to the very sensitive detection sensors of the BES-III spectrometer while they made sure that everything is working as expected. The next day, intensities were increased and a ten-times higher collision rate was measured. Over the next several weeks the intensity of the beams will gradually be further increased while at the same time BES-III‘s nearly 20,000 detection elements will be carefully adjusted and calibrated. When this process is completed, sometime in the early Fall, the BES-III research program will begin.